I followed this walkthrough on how to select colors with anti-aliasing, and when I finished, everything that hadn't been deleted was not opaque. Yet the layer was at 100% Opacity. How do I fix this??
The text really seems to be partially transparent - not only at the edges due the anti-aliasing, but whole letters. Color to alpha has affected also partially in the middle of the letters. I guess you have tried to get rid of the remnants of the green color with the treshold sliders in Color to Alpha dialog.
You should accept a little green to remain, then you can get more solid letters. You can desaturate the remnants of the green after Color to Alpha is applied.
A fix: make a few duplicates of the text layer and merge them. Partially transparent parts cumulate to more opaque. Desaturate off the remnants of the green, because also green gets stronger with layer duplication.
Fuzzy select the background with the default threshold. Shift-click inside closed areas ('a','e','o') to add them to the selection. In your very specific case (only two constrasting colors in the image), you also do a color selection and use the background color (so yo udon't have to worry about closed areas)
Select>Grow by one pixel so that the selection extends over the pixels on the edges of the letters (if the image is dirty (JPEG artifacts), you can grow by 2 or 3px)
- Colors>Color to alpha and remove the background color. With the selection above, C2A will only apply to edge pixels and leave the core of the letters untouched.